A conventional electric power steering control device includes steering torque detection means for detecting a steering torque applied by a driver, a torque controller for computing an assist torque current for assisting the steering torque based on the detected steering torque signal, a motor for generating a torque for assisting the steering torque, rotation speed estimating means for estimating a rotation speed of the motor, a damping controller for computing a damping current to be added to the assist torque current by using an estimated value of the estimated motor rotation speed, motor rotation angle detection means, and motor current detection means. The rotation speed estimating means includes steering component removing means for removing a component due to steering from the detected value of the motor rotation angle and the motor current detection means, and a rotation speed observer configured for a vibration equation having an inertia moment of the motor as an inertia term and a rigidity of a torque sensor as a spring term, the rotation speed observer being for computing the estimated value of the motor rotation speed based on the motor rotation angle and the motor current which are output from the steering component removing means (for example, Patent Document 1).
Patent Document 1: JP 2000-168600 A (Page 10, FIG. 12)